"If your browser's user agent string is 'xmlset_roodkcableoj28840ybtide' (no quotes), you can access the web interface without any authentication and view/change the device settings," he wrote Saturday in a blog post.

When read in reverse, the last part of this hard-coded value is "edit by 04882 joel backdoor."

D-Link will release firmware updates to address the vulnerability in affected routers by the end of October, the networking equipment manufacturer said via email.

The company did not clarify why the backdoor was placed in the firmware in the first place or what router models are affected.

According to Heffner, the affected models likely include D-Link's DIR-100, DI-524, DI-524UP, DI-604S, DI-604UP, DI-604+, TM-G5240 and possibly DIR-615. The BRL-04UR and BRL-04CW routers made by Planex Communications might also be vulnerable because they also appear to use the same firmware, he said.

The risk of unauthorized access is higher for routers that have been configured for remote management and have their Web administration interface exposed to the Internet.

However, even when the interface is only accessible from the internal network -- the default setting in D-Link routers -- this backdoor can still pose a threat because any visitor who connects to the wireless network or any piece of malware running on a computer inside the network can exploit it to make unauthorized changes to the router's configuration.

Such changes can have serious security consequences. For example, changing the DNS (Domain Name System) servers used by the router -- and inherently every device on the network -- with DNS servers controlled by an attacker would enable the attacker to redirect users to rogue websites when trying to access legitimate ones.

"Owners of affected devices can minimize any potential risk by ensuring that their router has the Wi-Fi password enabled and that remote access is disabled," D-Link said.

"If you receive unsolicited e-mails that relate to security vulnerabilities and prompt you to action, please ignore it," the company said. "When you click on links in such e-mails, it could allow unauthorized persons to access your router. Neither D-Link nor its partners and resellers will send you unsolicited messages where you are asked to click or install something."